The doubly labeled water (DLW) method of measuring total energy expenditure (TEE) has emerged as a vital tool in obesity research. It has enabled researchers to assess human energy requirements in health and in disease, to study the etiology of obesity, and to validate tools for the assessment of dietary intake. Picarro proposes to develop novel instruments that will measure oxygen and hydrogen isotope ratios in water with high precision by cavity ring down spectroscopy (CRDS). Currently DLW analysis is performed by only 3 or 4 laboratories in the US, and DLW researchers must send their samples to one of these facilities for analysis. As compared with existing mass spectrometry techniques, the proposed instruments will reduce the cost and difficulty of DLW studies, thus enabling many researchers to perform DLW analysis in their own laboratories. They will be able to perform more research, more quickly, conveniently, and at lower cost. In addition, a field-ready instrument not requiring extensive sample preparation would greatly extend the application's reach of energy expenditure methodology. The instrument could also be used by manufacturers of isotopically enriched water for quality control of their product. The doubly labeled water (DLW) method of measuring total energy expenditure (TEE) has emerged as a vital tool in obesity research. The proposed instruments will reduce the time, cost, and difficulty of DLW studies, thus enabling many researchers to perform DLW analysis in their own laboratories. A field-ready instrument not requiring extensive sample preparation would greatly extend the application's reach of energy expenditure methodology. [unreadable] [unreadable] [unreadable]